Tire air pressure detecting device

ABSTRACT

It is an object of the present invention to detect a tire air pressure indirectly with high detection precision. 
     A tire air pressure detecting device includes a speed sensor for outputting a signal corresponding to a rotational speed of a tire, and an electronic control unit which inputs the signal from the speed sensor and performs predetermined arithmetic operations on that signal. The electronic control unit calculates a wheel speed based on the output signal of the speed sensor, performs a frequency analysis for the calculated vehicle speed, and derives a resonance frequency corresponding to the unsprung mass in the vertical and longitudinal directions. The tire air pressure is then detected based on this resonance frequency.

This is a continuation-in-part application of application Ser. No.08/087,703 filed Jul. 9, 1993 now abandoned, which, in turn, is based onPCT application PCT/JP92/01457 filed Nov. 10, 1992.

TECHNICAL FIELD

The present invention relates to an air pressure detecting device whichdetects an air pressure condition in a vehicular tire.

BACKGROUND ART

Conventionally, as a device for detecting an air pressure in a vehiculartire, there has been proposed a direct detection device which uses apressure responsive member within the tire. However, because thepressure responsive member must be provided within the tire, this deviceresults in complicated construction and high costs.

Therefore, there has been proposed a device for indirectly detecting thetire air pressure which is based on a detection signal from a wheelspeed sensor. This detection signal represents a wheel speed for eachwheel which is based on a relationship between the tire radius and thetire air pressure. For instance, when the tire radius becomes smaller,the tire air pressure is decreased.

The tire radius may be affected by differences in each tire which aredue to tire wear or traveling conditions such as cornering, braking,starting or so forth. Furthermore, the radius of many tires does notchange responsively to changes in the tire air pressure. For instance,when the tire pressure is decreased at a rate of 1 Kg/cm², thecorresponding tire radius deformation magnitude may be only 1 mm. Forthese reasons, the method for indirectly detecting the tire air pressurebased on the deformation magnitude in the tire radius, is problematic inthat it cannot always provide accurate tire air pressure detection.

SUMMARY OF THE INVENTION

The present invention is presented to provide accurate tire air pressuredetection in view of the problems set out above. Accordingly, it is anobject of the present invention to provide a tire pressure detectingdevice which indirectly detects the tire air pressure with highdetecting precision.

In order to accomplish the above-mentioned object, a tire air pressuredetecting device, according to the present invention, detects the airpressure of a tire by generating, monitoring, and adjusting to changesin a signal which contains a vibration frequency component correspondingto the tire. Variations within a tire vibration frequency pattern aredetermined based on that signal. More specifically, when the tire airpressure varies, the associated tire spring constant also varies.Consequently, because the tire vibration frequency component pattern inthe signal containing the tire vibration frequency component is variedthrough spring constant variation, the tire air pressure condition canbe determined based on the variation of this pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing the orientation of the firstembodiment of the invention;

FIG. 2 is a characteristic chart showing a frequency characteristics ofacceleration for an unsprung mass of a vehicle;

FIG. 3 is a characteristic chart showing variation of resonancefrequencies of the unsprung mass of the vehicle due to variation of atire air pressure in vertical and longitudinal directions;

FIG. 4 is an illustration of the principle of detection of the tire airpressure in the first embodiment;

FIG. 5 is a chart showing a waveform of an output voltage of a wheelspeed sensor;

FIG. 6 is a chart showing a waveform showing a varying condition of awheel speed v which is calculated on the basis of a detection signalfrom the wheel speed sensor;

FIG. 7 is a characteristic chart showing a result of frequency analyzingoperation with respect to wheel speed v of the waveform illustrated inFIG. 6;

FIG. 8 is an illustration of an averaging process of the firstembodiment;

FIG. 9 is a characteristic chart showing a result of frequency analysisafter a moving averaging process in the first embodiment:

FIG. 10 is a flowchart showing a process of an electronic control unitof the first embodiment;

FIG. 11 is a characteristic chart showing a relationship between thetire air pressure and the resonance frequencies in the second embodimentof the invention;

FIG. 12 is a flowchart showing the difference in process between thesecond embodiment and the first embodiment;

FIG. 13 is an illustration showing a construction of the thirdembodiment of the invention;

FIG. 14 is a flowchart showing the difference in process between thethird embodiment and the first embodiment;

FIG. 15 is an illustration showing a construction of the fourthembodiment of the invention;

FIG. 16 is an illustration showing a construction of the fifthembodiment of the invention;

FIG. 17 is a flowchart showing a process of the electronic control unitof the sixth embodiment;

FIG. 18 is a timing chart showing a variation of the wheel speed;

FIG. 19 is a characteristic chart illustrating peaks in gain whichcorrespond to the integral multiples of wheel rotations per unit time;

FIG. 20 is an illustration of the control performed by the seventhembodiment;

FIG. 21 is a flowchart illustrating a principle of the process of theseventh embodiment;

FIG. 22 is an illustration of the control performed by the eighthembodiment;

FIG. 23 is a flowchart illustrating a principle of the process of theeighth embodiment;

FIG. 24 is an illustration of the control performed by the ninthembodiment;

FIG. 25 is a flowchart illustrating a principle of the process of theninth embodiment;

FIGS. 26(a), 26(b) and 26(c) are illustrations of the principle of theprocess the tenth embodiment;

FIG. 27 is a flowchart illustrating a principle of the process of thetenth embodiment;

FIGS. 28(a) and 28(b) are illustrations of the control performed by theeleventh embodiment;

FIG. 29 is a characteristic chart showing a frequency distribution ofthe wheel speed in the eleventh embodiment;

FIG. 30 is a characteristic chart showing predicted gain distribution ofa tire rotation degree component in the eleventh embodiment;

FIG. 31 is a characteristic chart showing a frequency characteristicsfrom which the tire rotation degree component is removed in the eleventhembodiment;

FIG. 32 is a flowchart illustrating a principle of the process of theeleventh embodiment;

FIGS. 33(a), 33(b) and 33(c) are illustrations of the control performedby the twelfth embodiment;

FIG. 34 is an illustration for discussion of an outline of the controlperformed by the twelfth embodiment;

FIG. 35 is an illustration for discussion of an outline of the controlperformed by the twelfth embodiment;

FIG. 36 is a flowchart illustrating the first portion of the principleof the process of the thirteenth embodiment;

FIG. 37 is a flowchart illustrating the second portion of the principleof the process of the thirteenth embodiment;

FIG. 38 is a characteristic chart showing a relationship between avehicle speed ratio and a gain coefficient;

FIG. 39 is a flowchart illustrating a principle of the process of thefourteenth embodiment;

FIG. 40 is a characteristic chart showing a relationship between avehicle speed and a gain of respective degrees of the frequencycorresponding to the wheel rotation speed per unit time;

FIG. 41 is a flowchart illustrating a principle of the process of thefifteenth embodiment;

FIG. 42 is a flowchart illustrating a first portion of the principle ofthe process of the sixteenth embodiment;

FIG. 43 is a flowchart illustrating a second portion of the principle ofthe process of the sixteenth embodiment;

FIG. 44 is a flowchart illustrating a first portion of the principle ofthe process of the seventeenth embodiment;

FIG. 45 is a flowchart illustrating a second portion of the principle ofthe process of the seventeenth embodiment;

FIG. 46 is a characteristic chart showing a relationship of the numberof data (SMP) in relation to an difference Δf between a resonancefrequency f_(k) and a discriminated value f_(L) ;

FIG. 47 is a characteristic chart showing a relationship of the numberof an averaging process (SUM) with respect to a difference Δf between aresonance frequency f_(k) and a discriminated value f_(L) ;

FIG. 48 is a flowchart showing a first portion of the principle of theprocess in the eighteenth embodiment;

FIG. 49 is a flowchart showing a second portion of the principle of theprocess in the eighteenth embodiment;

FIG. 50 is a flowchart showing a principle of the process in thenineteenth embodiment;

FIGS. 51(a) and 51(b) are charts showing waveforms of the vehicle speedin a time sequence calculated by ECU;

FIG. 52 is a characteristic chart showing a relationship between a wheelspeed variation magnitude Δv and the number of data (SMP);

FIG. 53 is a characteristic chart showing a relationship between a wheelspeed variation magnitude, Δv, and the number of the averaging processes(SUM);

FIG. 54 is a flowchart showing the process of the electronic controlunit of the twentieth embodiment;

FIG. 55 is a timing chart showing a relationship between the wheel speedand resonance frequency in the twentieth embodiment;

FIG. 56 is a flowchart showing the principle of the process of thetwenty-first embodiment;

FIGS. 57(a) and 57(b) are characteristic charts showing a relationshipbetween the wheel speed, the tire air pressure and resonance frequencyof the unsprung mass;

FIG. 58 is a characteristic chart showing a relationship between thetire pressure of the radial tire and stadless tire and resonancefrequency in the unsprung mass;

FIG. 59 is a flowchart showing a process of the ECU of the twenty-secondembodiment;

FIG. 60 is a flowchart showing a process of the ECU of the twenty-secondembodiment;

FIG. 61 is a flowchart showing a process of the ECU of the twenty-thirdembodiment;

FIG. 62 is an illustration of the relationship between the tire airpressure and the resonance frequency and the tire air pressure;

FIG. 63 is an illustration showing an orientation of a tire air pressuredetecting device which includes a set switch;

FIG. 64 is a flowchart of the process of the ECU of the twenty-fourthembodiment;

FIG. 65 is a graph showing a relationship of an effective rolling radiusand the resonance frequency of the unsprung mass;

FIG. 66 is a first portion of a flowchart of the signal processing ofthe electronic control unit in the twenty-fifth embodiment;

FIG. 67 is a second portion of a flowchart of the signal processing ofthe electronic control unit in the twenty-fifth embodiment.

FIG. 68 is a graph showing a relationship of the tire air pressure andthe resonance frequency of the unsprung mass;

FIG. 69 is a graph showing a relationship of an effective rolling radiusand the resonance frequency of the unsprung mass;

FIG. 70 is a first portion of a flowchart of the signal processing ofthe electronic control unit in the twenty-sixth embodiment;

FIG. 71 is a second portion of a flowchart of the signal processing ofthe electronic control unit in the twenty-sixth embodiment;

FIG. 72 is a characteristic chart illustrating fluctuation of the tireair pressure with respect to the same resonance frequencies based on theunsprung mass load;

FIG. 73 is a characteristic chart showing a relationship betweenresonance frequency difference and the tire air pressure;

FIG. 74 is a characteristic chart showing a relationship betweenresonance frequency f_(MAX) and the resonance frequency differencef_(TH) ;

FIG. 75 is a characteristic chart showing a relationship betweenresonance frequency f_(MAX) and the resonance frequency differencef_(TH) ; and

FIG. 76 is a flowchart of the signal processing of the electroniccontrol unit in the twenty-seventh embodiment;

FIG. 77 is a characteristic diagram showing the relationship between thetire pressure and the unsprung resonance frequency for the radial tireand the stadless tire;

FIG. 78 is a flowchart of the process performed in the twenty-eighthembodiment;

FIG. 79 is a characteristic diagram showing the relationship between theresonance frequency and the wheel speed (gain);

FIGS. 80(a) and 80(b) contain various characteristic diagrams of therelationship between the resonance frequency and the wheel speed (gain);

FIG. 81 is a characteristic diagram demonstrating that the relationshipbetween the resonance frequency and the wheel speed (gain);

FIG. 82 is a characteristic diagram demonstrating that the relationshipbetween the tire pressure and the judgment resonance frequency;

FIG. 83 is a flowchart showing the processes performed in thetwenty-ninth embodiment;

FIGS. 84(a), 84(b) and 84(c) are illustrations of the data selectionprocess in the twenty-ninth embodiment;

FIG. 85 is an illustration of the gain adjustment in the twenty-ninthembodiment;

FIG. 86 is a flowchart showing some of the processes performed by andECU in the thirtieth embodiment;

FIG. 87 is an illustration of the data adjustment performed in thethirtieth embodiment; and

FIG. 88 is an illustration of the gain adjustment process performed inthe thirtieth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be explained hereinafter with reference tothe drawings.

As shown in the overall construction of the first embodiment illustratedby FIG. 1, wheel speed sensors are provided for each tire 1a˜1d of avehicle. Each wheel speed sensor comprises gears 2a˜2d and pick-up coils3a˜3d. The gears 2a˜2d are coaxially mounted on a rotary shaft (notshown) of each tire 1a˜1d, and are made from disc-shaped magneticbodies. The pick-up coils 3a˜3d are positioned in close proximity to thegears 2a˜2d with a predetermined gap therebetween for outputting analternating current (AC) signal which has a period corresponding to therotational speed of both gears 2a˜2d and tires 1a˜1d. The AC signaloutput from pick-up coils 3a˜3d is input into a known electronic controlunit (ECU) 4 comprising a wave shaping circuit, ROM, RAM and so forth. Apredetermined signal processing, which includes wave shaping, isperformed. The result is input into display portion 5 which indicatesthe air pressure condition of each tire, 1a˜1d, to the driver. Thedisplay portion 5 may either display the air pressure condition of eachtire independently, or it may provide an alarm lamp which is turned onwhen the air pressure of a tire is lowered below a reference airpressure.

The tire air pressure detecting device will now be explained accordingto the present embodiment.

When a vehicle travels on a paved asphalt road, for example, the tire issubject to upward and downward (also known as vertical), forward andbackward (also known as longitudinal) forces due to the fine undulationon the road surface. These cause the tire to vibrate, where forward andbackward references will be referred to as longitudinal, and upward anddownward references will be referred to as vertical throughout theremainder of the specification.

FIG. 2 shows a frequency characteristic of the acceleration of theunsprung mass of a vehicle during tire vibration. As shown in thisfigure, the frequency characteristic of the acceleration has peak valuesat two points. Point `a` represents the resonance frequency of theunsprung mass of the vehicle in vertical directions, and the point `b`represents the resonance frequency of the unsprung mass of the vehiclein the longitudinal directions.

Since the rubber portion of the tire has a spring constant, the verticaland longitudinal resonance frequencies are both varied with variationsin tire air pressure. For instance, as shown in FIG. 3, when the tireair pressure is lowered, the spring constant of the rubber portion ofthe tire is also lowered. Consequently, the resonance frequencies arelowered in both the vertical and longitudinal directions. Accordingly,by extracting at least one of the resonance frequencies in thosedirections from the tire vibration frequency, the corresponding tire airpressure condition can be detected.

Therefore, in the present embodiment, the resonance frequenciescorresponding to the vertical and longitudinal directions of theunsprung mass of the vehicle are extracted from a detection signal ofthe wheel speed sensor. This is because, as a result of extensive studymade by the inventors, it has been found that a detection signal of awheel speed sensor includes a frequency component of tire vibration.Namely, as a result of frequency analysis of the wheel speed sensordetection signal, it has been determined that two peak values exist asshown in FIG. 4, and that the two peak values decrease in magnitude whenthe tire air pressure is lowered.

In recent years, an increasing number of vehicles have been equippedwith anti-skid control systems (ABS). Since these systems already havewheel speed sensors for each tire, tire air pressure within those tirescan be detected without any additional sensors. It should be noted thatbecause most resonance frequency variations are directly related to tirespring constant variations which are due to variations in tire airpressure, tire air pressure can be stably detected without concern forother tire factors, such as wearing and so forth.

In FIG. 10, there is illustrated a flowchart which shows a process thatis executed by ECU 4. Although ECU 4 performs similar processes forwheels 1a˜1d, the flowchart of FIG. 10 shows the flow of the processwith respect to a single wheel. Further, in the explanation givenhereinafter, suffixes for respective reference numerals are omitted. Inthe flowchart shown in FIG. 10, there is illustrated a particularexample in which an alarm is provided for the driver when the airpressure of the tire decreases to a level below, or equal to, areference value.

In FIG. 10, at step 100A, wheel speed v is calculated by waving shapingthe AC signal output from pick-up coil 3 (shown in FIG. 5) to form apulse interval, and by dividing the pulse interval with the elapsedperiod therein. As shown in FIG. 6, wheel speed v normally contains alarge number of high frequency components including the vibrationfrequency component of the tire. At step 110A, variation magnitude Δv ofcalculated wheel speed v is compared to reference value v₀. If Δv isgreater than, or equal to v₀, the result is positive, and the process isadvanced to step 120A. At step 120A, the period ΔT, within whichvariation magnitude Δv of wheel speed v is held in excess of thereference value v₀, is compared a predetermined period, t₀. A positiveresult is obtained when ΔT is greater than, or equal to, t₀.

The processes of the above-mentioned steps 110A and 120A determinewhether the road surface permits detection of the tire air pressureaccording to the detection method of the present embodiment. Namely, inthe present embodiment, detection of the tire air pressure is performedon the basis of a variation of the resonance frequency contained in thevibration frequency component of the tire. Therefore, unless thevariation in wheel speed v is continuously greater than a certainmagnitude, sufficient data for calculation of the above-mentionedresonance frequency cannot be obtained. It should be noted that in thecomparison of step 120A, period ΔT corresponds to the period duringwhich variation magnitude Δv of wheel speed v is equal to, or greaterthan, reference value v₀. Also, measurement of period ΔT is continuedwhen variation magnitude Δv of wheel speed v is again equal to orgreater than reference value v₀.

If steps 110A and 120A produce positive results, the process is advancedto step 130A. On the other hand, if either one of steps 110A and 120Aare negative, the process returns to step 100A.

At step 130A, a frequency analyzing operation (FFT) is performed withrespect to the calculated wheel speed so that the cycles of operation Nare counted. One example of the result of FFT operation is shown in FIG.7.

As shown in FIG. 7, substantially random frequency characteristics aretypically obtained when the FFT operation is performed with respect tothe wheel speed obtained on normal roads. This is a result ofirregularities in the configuration (size and height) of the smallundulations on road surface. Accordingly, the frequency characteristicsmay vary for each wheel speed data.

The present embodiment is directed at suppressing variations of thefrequency characteristics by deriving an average value which correspondsto the results of FFT operation over many operation cycles. At step140A, the number of the FFT operation cycles, N, is compared to thepredetermined number, n₀. If the number of operation cycles does notreach the predetermined number n₀ of cycles, the processes at steps 100Athrough 130A are again executed. On the other hand, when the number ofoperation cycles does reach the predetermined number n₀ of cycles, theprocess is advanced to step 150A to perform an averaging process.

As shown in FIG. 8, the averaging process is used to derive an averagevalue of the respective FFT operation results, from which an averagevalue of gains of respective frequency components are derived.Therefore, this averaging process is used to reduce the FFT operationresults according to the road surface being traveled.

However, the above-mentioned averaging process may be problematicbecause of noise the maximum peaks exist in the vertical andlongitudinal directions which do not always represent the resonancefrequency gains. To overcome this potential problem, the movingaveraging process set out below is performed at step 160A of the presentembodiment in lieu of the foregoing averaging process.

The moving averaging process of step 160A is performed by deriving gainY_(n) at an nth frequency using the following equation:

    Y.sub.n =(Y.sub.n+1 +Y.sub.n-1)/2                          (1)

Namely, in the moving averaging process, gain Y_(n), corresponding tothe nth frequency, is derived as an average value of the (n+1)thfrequency, which is the result of operation in the preceding cycle, andthe (n-1)th frequency which was previously derived. The result of theFFT operation is a smoothly varying waveform, and the results of themoving average process are shown in FIG. 9.

It should be noted that the wave shaping process is not specified in theforegoing moving averaging process. A low-pass filter can be employed toobtain the results of the FFT operation.

Alternatively, it is possible to perform a differentiating operation todetermine the wheel speed v before performing the FFT operation at step130A. Then, the FFT operation may be subsequently performed to determinethe result of the differentiating operation.

At step 170A, based on the results of the FFT operation which aresmoothed through the above-mentioned moving average process, resonancefrequency f_(K) of the unsprung mass in the longitudinal direction isderived. Then, at step 180A, lowering difference (f₀ -f_(K)) is derivedfrom initial frequency f₀ which is preliminarily set based on the normaltire air pressure. This lowering difference is compared with apredetermined difference Δf, where predetermined difference Δfcorresponds to an allowable lowest value (e.g. 1.4 kg/m²) of the tireair pressure with reference to the initial frequency f₀. Accordingly, ifdetermination is made at step 180A that lowering difference (f₀ -f_(K))is equal to, or greater than, predetermined difference Δf, the tire airpressure is regarded to be below the allowable lowest value. In thiscase, the process is advanced to step 190A and an alarm is displayed forthe driver on display portion 5.

It should be noted that although in the foregoing embodiment, an exampleis illustrated to detect decreases is the tire air pressure based onresonance frequency in the longitudinal direction, it is also possibleto detect the tire air pressure based on the resonance frequency in thevertical direction, or based on resonance frequencies in both thelongitudinal and vertical directions.

It should also be noted that not all of the ECU processes performed inthe following embodiments differentiate from those in the firstembodiment. Similarly, some aspects of the construction and orientationof the components used in the following embodiments are generally commonto previously discussed embodiments. Therefore, only those processes,components and orientations of the following embodiments which aredifferent from previously discussed embodiment will be described.

In the second embodiment, abnormal tire air pressures are determined bydirectly comparing the air pressures of the tires with predetermined airpressure value P_(o). Therefore, instead of using a frequency value forcomparison, such as the lowering difference (f_(o) -f_(k)) of the firstembodiment, this embodiment uses the derived tire air pressure value.

Accordingly, in the second embodiment, step 180A of the first embodimentis replaced by steps 182B and 184B of FIG. 12.

In step 182B, a relationship between tire air pressure and resonancefrequency (as shown in FIG. 11) is used to derive the actual tire airpressure from the detected resonance frequency. Then, at step 184B, thederived tire air pressure is compared with allowable minimum value P_(o)of the preliminarily set tire air pressure.

When derived air pressure P is below allowable minimum value P_(o), theprocess is advanced to step 190A where an alarming process is performed.Otherwise, the process is returned to step 100A where the detectionprocess is repeated.

It should be noted that, in the second embodiment, the tire air pressurederived in step 182B may be directly displayed on the display portion.

In the third embodiment, acceleration sensor 11 is positioned on anunsprung mass member of the vehicle and is used to generate a signalcontaining the vibrational frequency component of the tire.

Accordingly, as shown in FIG. 13, the output generated by wheel speedsensor of the first embodiment is replaced with that of accelerationsensor 11 of the embodiment. Correspondingly, the process shown in FIG.14 is executed in lieu of step 100A of the flowchart of FIG. 10.Specifically, step 102 discloses reading an acceleration signal fromacceleration sensor 11 for further processing.

Because it is possible to derive the resonance frequencies in both thevertical direction and the longitudinal direction by performing an FFToperation on the acceleration of the unsprung mass of the vehicle, anFFT operation may be applied directly to the output signal produced byacceleration sensor 11.

Consequently, the third embodiment does not require the wave shapingoperations which were needed to condition the output of the wheel speedsensors of the first embodiment.

In the fourth embodiment of the present invention, vehicle heightsensors are used for detecting a relative displacement between the bodyof the vehicle (sprung mass member) and the tire (unsprung mass member).In this method, vehicle height sensor 20 is used to generate an outputcontaining the tire vibration frequency component, thereby replacing thewheel speed sensors of the first embodiment.

Specifically, the vehicle height sensor, shown in FIG. 15, generates anoutput detection signal which is subjected to an appropriate low-passfiltering process. The resultant signal then undergoes twodifferentiating processes to develop a signal which is representative ofa relative acceleration between the vehicle body and the tire. Finally,this signal is produced as an output which is operated on in accordancewith steps 110A-190A of FIG. 10.

In the fifth embodiment of the present invention, load sensor 30replaces wheel speed sensor 20 to generate an output signal containingthe tire vibration frequency component. Similar to height sensor 20 ofthe fourth embodiment, load sensor 30 detects, and generates, a signalcorresponding to a load between the vehicle body (sprung mass member)and the tire (unsprung mass member).

As shown in FIG. 16, load sensor 30 is disposed within a piston rod of ashock absorber, and it comprises a piezoelectric element for generatinga signal whose amplitude corresponds to the applied load. Load sensor 30then outputs a signal which corresponds to a damping force of the shockabsorber. The tire air pressure can be detected by performing signalprocessing on this output signal, as performed by the foregoing thirdembodiment.

As a result of experiments made by the inventors, it has been found thatsignals which contain the actual tire vibration frequency component alsocontain noise corresponding to unbalances within the tire, where unevenwearing, standing wave phenomenon and so forth may cause suchunbalances. Furthermore, this noise occurs at integral multiples offrequency which correspond to both the number of rotations of the wheelduring a unit period of time, and the resonance frequency of theunsprung mass in the vertical or longitudinal directions.

Because the resonance frequency of the unsprung mass in the vertical orlongitudinal direction is extracted from the signal containing the tirevibration frequency component, it is often erroneously derived in theabove-mentioned embodiments. The accuracy of detection for tirefrequency and air pressure, which are determined using theabove-described embodiments, is therefore undeterminable. For thisreason, improvements in the detection accuracy are sought.

The sixth to fifteenth embodiments are set forth to achieve improvementsfor the accurate detection of resonance frequency and tire air pressureby overcoming and compensating for the problem set forth above.

In the sixth embodiment, the processes shown in FIG. 17 are performed,where steps 1000F to 1200F are similar to steps 100A to 120A of thefirst embodiment.

However, at step 1300F of the sixth embodiment, wheel speed variationratio A is derived on the basis of variation magnitude Δv₂, where Δv₂ isbased on wheel speed v within the predetermined period t₀₂ (t₀₂ >>ΔT),as shown in FIG. 18.

    A=Δv.sub.2 /t.sub.02                                 (2)

Then, at step 1400F, wheel speed variation ratio A is compared withpredetermined value A₀.

This comparison is made to determine whether the tire air pressure isaccurately detectable via the method of the present embodiment. Namely,when variation Δv₂ is small, the peaks (herein after referred to as"tire rotation degree components") appear at integral multiples of afrequency, where the frequency corresponds to the number of the wheelrotation per unit period, as shown in FIG. 19.

Therefore, unless wheel speed v varies above a certain magnitude withinthe predetermined period, the tire rotation degree component cannot beremoved, and the tire air pressure cannot be accurately determined.

Accordingly, when wheel speed variation ratio A is determined to besmaller than predetermined value Δ₀, the process returns to step 1000F.However, if it is determined in step 1400F that wheel speed variationratio A is equal to, or greater than, predetermined value A₀, then theprocess is advanced to steps 1500A˜1900A where processes similar tothose of the first embodiment are performed.

Then, at step 2000F, resonance frequency f_(K) is compared with theupper and lower limit values f_(H) and f_(L) of the resonance frequencyof the unsprung mass, where upper limit value f_(H) and lower limitvalue f_(L) are set corresponding to allowable upper and lower limitvalues of the tire air pressure (e.g. upper limit value is 2.5 kg/cm²and the lower limit value is 1.4 kg/cm²). When resonance frequency f_(K)is determined to be equal to, or greater than, upper limit value f_(H),the tire air pressure is regarded as being in excess of the allowableupper value. When the resonance frequency f_(K) is equal to, or lowerthan, lower limit value f_(L), the tire air pressure is regarded to belower than the allowable lower limit value. In either case, the processis advanced to step 2001F to perform alarming display to the driver viadisplay portion 5.

Thus, in the sixth embodiment, the FFT operation is used to derive thetire vibrational frequency component only when wheel speed variationratio A is equal to, or greater than, predetermined value A₀. For thisreason, the tire rotation degree component, which appears while thespeed variation ratio A is small, can be eliminated.

In the seventh embodiment, the processes of steps 1300F and 1400F of thesixth embodiment are replaced with steps 1310G and 1311G of FIG. 21.

At step 1310G, variation magnitude Δv₃ is derived based on wheel speed vwithin unit period t₀₃. At step 1311G, variation magnitude Δv₃(N) iscompared to each previous variation magnitude Δv₃ (1)˜Δv₃ (N-1), wherevariation magnitude Δv₃(N) is derived in the Nth cycle of step 1310G,and where Δv₃ (1)˜Δv₃ (N-1) are derived in the 1st˜(N-1)th cycles ofstep 1310G.

If current variation magnitude Δv₃(N) is equal to any of the previousvariation magnitudes, the process is returned to step 1000F, and no FFToperation is performed.

However, when Δv(N) is not equal to any of the previous variationmagnitudes, the process is advanced to step 1500A to perform the FFToperation. It follows that the tire vibration frequency component, whichis subject to the FFT operation, has unique wheel speed variationmagnitudes Δv₃ corresponding to each cycle.

For example, as shown in FIG. 20, the variation magnitude calculated inthe third cycle, Δv₃(3), is equal to previously calculated variationmagnitude from the first cycle, Δv₃ (1). Therefore, in the third cycle,Δv₃ (3) would be eliminated and the process would be returned to 1000Fwithout further processing. Alternatively, in cycles where the variationmagnitude is not equal to any previously calculated variationmagnitudes, the processing is advanced to step 1500F so that an FFToperation and subsequent averaging processes may be performed.

Therefore, the peak appearing in the tire vibration frequency componentmaintains the resonance frequency component of the unsprung mass inlongitudinal and vertical directions at the same frequency. However,tire rotation frequency components appear at different frequencies, areremoved by the FFT operation performed in step 1500F and subsequentsteps.

Thus, although the FFT operation is performed when the current variationmagnitude is different from any of the previous variation magnitudes,another criteria exists. Namely, the FFT operation is performed onlywhen average wheel speed v_(C)(N) is different from any previous averagewheel speed, v_(c)(1) ˜v_(c)(N-1), where v_(C)(N) is derived in duringthe Nth operation and v_(c)(1) ˜v_(c)(N-1) are derived prior to the Nthoperation.

In the foregoing sixth and seventh embodiments, the tire rotation degreecomponent is processed before the FFT operation is performed.

Alternatively, in the eighth embodiment, the tire rotation degreecomponent is processed after the FFT operation.

The gain of both the tire vibration frequency component and the tirerotation degree component are affected greatly by road surfacecondition. For instance, as shown in FIG. 22, when the vehicle travelson a rough or unpaved road, the gain of both the tire vibrationfrequency component and the tire rotation degree component become large.

Therefore, in the eighth embodiment, the FFT operation is used to derivetire vibration frequency components for each cycle. Then, the averagingprocess is performed only on those frequency cycles which have a maximumgain, v_(a), which falls within a predetermined range v_(Max) ˜V_(MIN).Because only consistent FFT operation results are considered by theaveraging process, the influence of the tire rotation degree componentafter the averaging process becomes small.

Accordingly, in the eighth embodiment, steps 1300F˜1600F of the sixthembodiment are replaced by steps 1320H-1323H which are illustrated inFIG. 23.

At step 1320H, an FFT operation is performed to derive the tirevibration frequency components. Then, at step 1321H, maximum gain fromamong the derived tire vibration frequency components, v_(a), iscompared to upper and lower limit values v_(MAX) and v_(MIN). If v_(a)is not between these limits, it is not used for averaging and theprocess returns to step 1000F. However, if v_(a) is between theselimits, it will be used for averaging and the process is advanced tostep 1322H where the number of values to be averaged, N_(A), isincremented.

Then, at step 1323H, the number of values to be averaged, N_(A), iscompared to predetermined value N_(B). If N_(A) is less than N_(B), theprocess is returned to step 1000F so that another FFT operation may beperformed. However, when N_(A) is greater than, or equal to,predetermined value N_(B), the process is advanced to step 1700F forcontinued processing.

Thus, data corresponding to travel over rough roads is removed so thatthe influence of the tire rotation degree component, which has largepeaks, can be suppressed.

The ninth embodiment features removing excessively large (or small) tirerotation degree components using a ratio, K_(i), between maximum gainv_(a) and predetermined gain v₀, where v_(a) corresponds topredetermined frequency band f_(b).

Namely, in the ninth embodiment, steps 1300F˜1500F of the sixthembodiment are replaced with steps 1330I˜1333I of FIG. 25.

In this embodiment, the FFT operation is performed at step 1330I. Atstep 1331I, a coefficient, K_(i), is obtained as the ratio of maximumgain value v_(a) to predetermined gain v₀, based on the FFT operationresult from step 1330I, where

    K.sub.i =v.sub.0 /v.sub.a                                  (3)

At step 1332I, the FFT operation results are corrected by multiplyingthe tire vibration frequency component derived in the FFT operation withcoefficient K_(i). After the FFT operation is completed, the counter forthe number of FFT operations performed is incremented at step 1333I, andthe process is advanced to step 1600F.

As shown in FIG. 24, this process has the effect of normalizing thegains by v₀ so that no excessively large (or small) data remains.

The tenth embodiment is directed at removing the tire rotation degreecomponent directly from the FFT operation result. Similar to the sixthembodiment, the tenth embodiment utilizes the fact that the tirerotation degree component is necessarily present within the frequencyrange which corresponds to the wheel speed variation range, or to anintegral multiple thereof.

For example, in FIG. 26(a), the wheel speed variation range within acertain period T_(D) falls within between minimum value a and maximumvalue b. Therefore, deriving frequencies A and B, corresponding to wheelspeeds a and b of FIG. 26(a), requires estimation of gain between thecorresponding frequency values, p and q. Thus, the portion between p andq, illustrated by the solid line of FIG. 26(b), is replaced with astraight line, illustrated by the solid line of FIG. 26(c). Such aseries of process is hereinafter referred to as "interpolation".

To perform this interpolation, steps 1300F˜1500F of the sixth embodimentare replaced with steps 1341J˜1344J of the tenth embodiment, illustratedin FIG. 27.

At step 1340J, minimum value a and maximum value b are derived, wherethey correspond to the wheel speed variation within a certain periodT_(D). At step 1341J, frequencies A and B, which correspond to foregoingminimum value a and maximum value b, are derived. Then, at step 1342J,the FFT operation is performed.

Because the tire rotation degree components are present within thefrequency range of A˜B, interpolation is performed by interpolatingbetween the FFT operation resultant values of frequencies A and B with astraight line at step 1343J, where the FFT operation resultant values atA and B are shown as q and p in FIGS. 26(b) and 26(c). Consequently, thegains corresponding to the tire rotation degree components of frequencyrange A˜B are reduced.

In step 1344J, the counter for indicating the number of cycles of theFFT operations performed is incremented, after which the process isadvanced to step 1600F for further processing.

The eleventh embodiment features improved precision for theinterpolation of the above-mentioned tenth embodiment. Namely, in thetenth embodiment, a distribution of wheel speed frequency (A_(i)) isderived for wheel speeds between maximum wheel speed value b and minimumwheel speed value a which are varying within period T_(D) (FIGS. 28 (a)and 28 (b)).

By sorting wheel speeds within wheel speed range of a˜b, and determiningthe number of data points corresponding to each wheel speed, adistribution of wheel speed frequencies may be derived. As discussedwith respect to the tenth embodiment, the tire rotation degree componentis necessarily present within the frequency range from A to B. However,it should be noted that the rotational degree component corresponds tothe wheel speed variations in the range from a˜b. Therefore, the gaindistribution of the tire rotation degree component is similar to thefrequency distribution of the wheel speed.

In other words, since the tire rotation degree component is apparentfrom the number of rotations of the wheel within a unit period, thewheel speed can be regarded as the number of rotations of the wheelwithin that unit period.

Therefore, coefficient K_(i) (which is the coefficient for convertingthe wheel speed frequency A_(i) into the FFT operated value v_(i) at thefrequency corresponding to the wheel speed) is multiplied with wheelspeed frequency A_(i) to predict the distribution of the gains of thetire rotation degree components (see FIGS. 29 and 30). Subsequently, asshown in FIG. 31, by subtracting the predicted distribution of gains ofthe tire rotation degree components from the result of the FFT operationwithin the frequency range of A˜B, the influence of the tire rotationdegree components are substantially decreased.

Correspondingly, by interpolating between the resultant values q and pof the FFT operation within the frequency range of A˜B, the rotationdegree components may also be substantially decreased.

The foregoing process is illustrated in the flowchart shown by FIG. 32,where steps 1340J˜1344J of the tenth embodiment are replaced with steps1350K˜1356K of FIG. 32.

At step 1350K, within the period T_(D), maximum wheel speed b andminimum wheel speed a are derived, and the results are stored in ECU 4.At step 1351K, the stored resultant wheel speeds are increasingly ordecreasingly sorted, and the number of equal speeds are calculated todetermine the distribution of the wheel speed frequency A_(i).

At step 1352K, the frequency corresponding to the wheel speed isderived. At step 1353K, the gains (νi) of the tire rotation degreecomponents are derived from the distribution of wheel speed frequency,A_(i) by multiplying coefficient K_(i) to A_(i).

Next, at step 1354K, the FFT operation is performed on the gains (νi) ofthe tire rotation degree components which were derived in step 1353K. Atstep 1355K, the gains of the tire rotation degree component (ν1) aresubtracted from the resultant value of the FFT operation (v_(i)) withinfrequency range A˜B, thereby generating a corrected value (v_(i) ')corresponding to the FFT operation. The resultant values of the FFToperation, from which the tire rotation degree components are removed,are as illustrated in FIG. 31.

At step 1356K, the counter for the number of FFT operation cyclesperformed is incremented. Then, the process is advanced to step 1600Ffor further processing as described in the sixth embodiment.

The twelfth embodiment features approximating the frequency distributionof the wheel speed with a convenient configuration, and subtracting theapproximated convenient configuration from the result of the FFToperation.

It should be noted that, as shown in FIGS. 33(a) and 33(b), the mannerused to derive the frequency distribution while the wheel speed variesfrom a to b, is the same as that of the eleventh embodiment.

However, this distribution may be approximated as follows. The mostfrequent wheel speed is shown as c. Therefore, the frequencydistribution is approximated by triangle abc' as shown in FIG. 33(c).Then, as shown in FIGS. 34 and 35, by multiplying predeterminedcoefficient K_(i) with the triangle abc', predicted gains (νi) of thetire rotation degree components are derived. Further, by subtracting thederived approximated gains from the resultant values (v_(i)) of the FFToperation, the tire rotation degree components are removed.

It should be noted that no flowchart is provided for this embodimentbecause the steps performed are substantially the same as that of theeleventh embodiment.

On the other hand, in the twelfth embodiment, by using the highestfrequency wheel speed c, an average value of the wheel speed variation,which is between values a and b may be determined without removing thetire rotation degree component. Further, instead of approximating thewheel speed frequency distribution with triangle abc', statisticaldistributions, such as normal distribution, Gaussian distribution and soforth may be employed.

The thirteenth embodiment is directed toward shortening the operationprocess period by determining the gains of the wheel rotation speeddegree components in units of time from a map. Namely, the referencedmap (FIG. 38) relates the initially derived gain of the degreecomponents of the wheel rotation speed per unit period (T) to a ratiobetween vehicle speed V of the subsequent process, and initially derivedvehicle speed V₀.

Explanation of this embodiment will be made with reference to FIGS. 36and 37.

Steps 101M˜104M are similar to those in the first embodiment. At step105M, vehicle speed V is derived based on the wheel speed used for theFFT operation process. Note that the vehicle speed, which is derivedimmediately after the initiation of process, is stored in the RAM asvehicle speed V₀. This value is derived to provide a center speedcomponent of wheel speed v in addition to the tire vibration frequencycomponent. At step 106M, judgment is made whether flag F is set to "1".

It should be noted that flag F is reset to "0" only in response to theturning OFF of an ignition switch. Therefore, in the first process afterturning ON the ignition switch, this flag is set to "0" so that anegative judgment is made in step 106M to advance the process to step107M.

At step 107M, vehicle speed V₀ is subject to a frequency conversion toobtain a primary frequency which corresponds to the number of rotationsof the wheel within a unit period. Additionally, integral multiples ofthe primary frequency are computed.

At step 108M, gains JV₁ ˜JV_(i) of the tire rotation degree componentsare read into the RAM based on the FFT operation results. At step 109M,flag F is set to "1", and the process is returned to step 101M.

Note that flag F is set to "1" so that the processes performed in steps107M and 108M are executed only once, immediately after starting.Accordingly, the process is directly advanced to step 110M in eachsubsequent cycle, where a vehicle speed rate (V/V₀) which is relative tothe vehicle speed V₀ is derived. At step 111M, gain coefficients K₁˜K_(i) are derived by reading a gain coefficient which corresponds tothe vehicle speed rate (V/V₀), from a map shown in FIG. 38, where themap is preliminarily stored in ECU 4.

At step 112M, gains dV₁ ˜dV_(i) are derived on the basis of both thedetermined gain coefficients K₁ ˜K_(i), and the gains of the tirerotation degree components JV₁ ˜JV_(i) from step 108M. At step 113M,gains dV₁ ˜dV_(i) are subtracted from the results of the FFT operationto eliminate the influence of the tire rotation degree components. Theprocesses following step 113M are similar to those in the foregoingembodiments.

In the fourteenth embodiment a predetermined relationship between gaindV and vehicle speed V_(x) is used to derive the gain at various wheelspeeds. The derived gain corresponding to each integral multiple of theprimary frequency degree is then subtracted from the results of the FFTcalculation so that the influence of the tire rotation degree componentmay be eliminated.

The foregoing process is illustrated in flowchart shown by FIG. 39,where steps 105M˜ 114M of the thirteenth embodiment are replaced withsteps 205N˜209N.

At step 205N, vehicle speed V_(X) is derived from the results of the FFToperation performed in step 104M of the thirteenth embodiment. At step206N, a frequency conversion is performed on derived vehicle speed V_(x)to determined the primary frequency of the tire rotation degreecomponent, and to obtain frequencies which correspond to the integralmultiples of the primary frequency. At step 207N, the gains whichcorrespond to respective degrees dV₁ ˜dV_(i) of vehicle speed V_(X), arederived from a map (FIG. 40) which is preliminarily stored in ECU 4. Atstep 208M, gains dV₁ to dV_(i) of the respective integral multiplefrequencies are subtracted from the results of the FFT operation,thereby eliminating the influence of the tire rotation degree component.

In the fifteenth embodiment, the frequency of the tire rotation degreecomponent is directly removed using a plurality of band-pass filters.This is explained with reference to the flowchart shown in FIG. 41.

In steps 301o to 303o, vehicle speed V_(X) is both derived from thewheel speed and frequency converted. Thus, the frequency range of thetire rotation degree component is obtained. At step 304o, a bandfrequency (f_(a) ˜f_(b)) of band-pass filter (B.P.F.) F₁ is used to setthe band frequencies of a plurality of band-pass filters, F₂ ˜F_(i).Specifically, the band frequency of band-pass filters F₂ ˜F_(i) arerespectively set as integral multiples of the band-pass frequency range,f_(a) ˜f_(b), Of band-pass filter F₁.

Then at step 305o, the original waveform is passed through therespective band-pass filters F₁ ˜F_(i) to obtain a time-based waveform,where the resultant waveform is not influenced by the tire rotationdegree components. Using this waveform, the FFT operational process andsubsequent averaging processes, which are explained with respect to thethirteenth and the fourteenth embodiments, are performed to deriveresonance frequency f_(K). Then, judgments can be made concerning thetire air pressure.

3sixteenth

The tire rotation degree component may also be removed by performing anFFT analysis on the filtered waveform and subtracting the results fromthe original waveform.

However, by using frequency analysis (FFT operation) to extract theresonance frequency, many summing and multiplying operations must beperformed, resulting in prolonged operation.

Therefore, the sixteenth to nineteenth embodiments are provided tomodify the FFT operation periods depending upon both the requiredresponse characteristics, and the required detection accuracy of thetire air pressure.

Foregoing FFT operations may have been performed by reading apredetermined amount of data into the RAM of ECU 4, and repeatingsumming and multiplying operations on all of this data so that theresonance frequency can be extracted. However, because the resonancefrequency is known in the present invention, a frequency range, w_(f),within which the FFT operation is performed can be preliminarily set.Accordingly, if a large amount of data is read into RAM of ECU 4, thefrequency range many be divided into many (n_(f)) smaller frequencyranges so that the frequency resolution (w_(f) /n_(f)) and detectingprecision are improved for each frequency range.

However, reading a large number of data into RAM, as set forth above,requires a longer period for obtaining one result of the FFT operation(hereinafter referred to as "FFT data"), and leads to a heavier load onECU 4.

Additionally, in order to achieve a high frequency resolution, a largenumber of FFT data must be provided for the averaging process.

Alternatively, when the required frequency resolution is lower, theamount of FFT data required for such averaging processes is reduced.

The sixteenth to nineteenth embodiments utilize the foregoing propertiesof the FFT operation.

For instance, when less detection accuracy is required, the number ofaveraging processes are reduced while the difference between the derivedresonance frequency and the reference value is large. Therefore, quickerresponse to relatively swift variation of the tire air pressure may beobtained by shortening the operation period of the FFT data.

On the other hand, when the tire air pressure is close to the referencevalue, the number of FFT data to be read into RAM is increased, therebyincreasing the number of the averaging process, the frequencyresolution, and the detection accuracy.

The processes of the sixteenth embodiment are illustrated by FIG. 42,where two different levels of specification values are determined foruse in the FFT operation. Accordingly, when the tire air pressureapproaches reference value f_(L), the specification values for the FFToperation are changed. The signal extraction period is expanded, andboth the amount of data to be sampled (SMP) and the number of theaveraging process cycles (SUM) are increased. Consequently, thefrequency resolution and tire air pressure detecting precision are alsoincreased, thereby decreasing the possibility of erroneous detection.

On the other hand, when the tire air pressure does not approachreference value f_(L), the tire air pressure detecting process isperformed in a shorter period under the specification of the FFToperation for lower frequency resolution and higher responsecharacteristics.

At step 101P of FIG. 42, specification values for the FFT operation areread, where the specification values for the operation are initially setfor lower detection accuracy.

At subsequent steps 102P˜105P, processes similar to those described insteps 100A˜130A of the first embodiment are performed.

At step 106P, the number of FFT operations performed (N_(S)) is comparedto a predetermined number (SNM). If the number of operation cycles hasnot reached the predetermined number, then steps 102P˜105P are repeated.0n the other hand, when the number of the operation cycles has reachedthe predetermined number, the process is advanced to perform theaveraging process at step 107P, moving averaging process at step 108P,and derivation of the resonance frequency f_(K) at step 109P.

Then, as shown at step 110P of FIG. 43, a difference between derivedresonance frequency f_(K) and predetermined air pressure loweringreference value f_(L) is derived, where the difference is referred to asΔf. At step 111P, the difference is compared to preset value f_(w) todetermine if the tire air pressure has decreased to a level nearreference value f_(L) so that an increase in detection accuracy isrequired.

When it is determined in step 111P that the difference is less thanf_(w), the number of sample data values (SMP), and the number ofaveraging process cycles (SUM), are increased to achieve higherdetection accuracy. At step 112P, the status of flag F is checked, wherethis flag is set to zero only once, namely the first time it is checkedafter initiation of process.

When the flag is set to zero, the process is advanced to step 113P toupdate the number of sample data to m_(L) and to update the number ofaveraging process cycles to N_(L), where m_(L) is less than m_(S), andwhere N_(L) is greater than N_(S). Then, at step 114P, flag F is set to"1", and the process is advanced to step 115P.

Otherwise, when either resonance frequency f_(k) is determined to not beclose to reference value f_(L) in step 111P, or when the status of flagF is determined to be equal to "1" in step 112P, no increase in thedetection accuracy is required. Accordingly, the process advancesdirectly to step 115P.

At step 115P, the most recent resonance frequency calculated, f_(K), iscompared to reference value f_(L). If resonance frequency f_(K) isdetermined to be lower than, or equal to, reference value f_(L), theprocess is advance to step 116P to generate an alarm indicatingdecreased tire air pressure. Otherwise, the process is returned to step102P to repeat the above-mentioned process.

After starting the process, if resonance frequency corresponding to thetire air pressure gradually approaches the reference value f_(L), andthe specification values of the FFT operation are updated, air pressurewill not be supplied to the tire until the vehicle stops.

Therefore, steps 112P and 114P are provided in order to avoid redundantprocessing of step 113P. Specifically, flag F is set to "1" in step 114Pto indicate that the specification values of the FFT operation have beenupdated. Thus, when it is detected in step 112P that the specificationvalues have been previously updated, these steps are not performed andthe air pressure lowering judgment process of step 115P is immediatelyperformed.

The seventeenth embodiment is directed at expanding the signalextraction period when Δf becomes smaller than f_(w2) so that both theSMP and the SUM are increased, where Δf is the difference betweenresonance frequency f_(K) and reference value f_(L), and f_(W) is apredetermined value. In this embodiment, the values of SMP and SUM whichcorrespond to Δf are determined by using predetermined relationships,such as those shown in FIGS. 46 and 47.

In other words, the difference Δf of resonance frequency f_(K) isrelated to both the number of data (SMP) shown in FIG. 45, and to thenumber of data (SUM) shown in FIG. 46, where each map is preliminarilystored in ECU 4. Therefore, values corresponding to SMP and SUM aredetermined on the basis of the FFT operation result.

Consequently, the number of levels for specification values of the FFToperation are increased, and further improvements in tire air pressuredetecting precision are achieved.

The foregoing process is illustrated in the flowchart shown by FIGS. 44and 45. In this flowchart, steps 112P˜114P of the sixteenth embodimentare replaced by step 211Q of the present embodiment.

Accordingly, at step 211Q, the specification of the FFT operation isupdated with values of SMP and SUM based on Δf which was derived in step210Q, where SMP is obtained from the map of FIG. 46 and where SUM isobtained from the map of FIG. 47. These values are then used to controlthe FFT operation as described with respect to the sixteenth embodiment.

It should be noted that, similar to step 101P of the sixteenthembodiment, step 201Q reads out the FFT operation specification valueswhich result in the lowest detecting precision. For instance, the lowestvalue for SMP is shown as DΔT4 in FIG. 46, and the lowest value for SUMis shown as N_(i) in FIG. 47.

The eighteenth embodiment is directed at decreasing the response timefor air pressure detection by decreasing FFT operation time. Thus, thealarm may be quickly engaged when the tire air pressure is decreased inthe acceleration state. This becomes particularly important when tireair pressure changes rapidly, such as when entering highways and thelike.

The eighteenth embodiment will be explained with reference to theflowcharts of FIGS. 48 and 49.

At steps 301R and 302R, the initial specification values for the FFToperation are read out and wheel speed v is calculated. At step 303R,the FFT operation and integration of the number of operation cycles areperformed.

Then, at step 304R, vehicle speed V and predetermined value V_(H) arecompared. If V is greater than, or equal to, V_(H), the process isadvanced to step 305R, where the status of flag F is checked. Becauseflag F is adapted to be reset only in response to turning OFF of theignition, the process is advanced to step 306R only after the firststatus check.

At step 306R, the period (T) in which vehicle speed V reaches set valueV_(H), is compared to a calculated period for performing an FFToperation using the initial specification values. Specifically, theperiod for performing an FFT operation using the initial specificationvalues is calculated as t×m_(s) ×N_(s), where t is a sampling period,m_(s) is number of data and N_(s) is the number of FFT data.

The process is advanced to step 307R when period T is less than, orequal to, the operational period, because this condition indicates thatthe period taken to reach set value V_(H) was shorter than the FFToperation period.

Normally, such cases frequently appear during an acceleration statebefore entry into high speed traveling. Accordingly, if the tire airpressure is low, the FFT operation must be quickly processed to generatea timely alarm indicating the state of low tire air pressure.

Therefore, at step 307R, a possible number N_(s) ' (truncated at radixpoint) is derived which corresponds to the maximum number of FFToperation cycles per period T. Then, at step 308R, the number of the FFTdata (SUM) is set equal to N_(s) '.

Flag F is set to "1" at step 309R, the averaging process is performed atstep 310R, the moving averaging process is performed at step 311R, andresonance frequency f_(K) is calculated at step 312R, where resonancefrequency f_(K) is performed based on the foregoing number N_(s) ' ofthe averaging process cycles.

Steps 313R and 314R are then performed to determine if the tire airpressure has been decreased below lower limit f_(L) and to display analarm if it has.

Note that when V is less than V_(H) at step 304R, period T is greaterthan the calculated operational period at step 306R, or when the flag isset equal to "1" in step 305R then the number of operation cycles iscompared to predetermined specification value SUM at step 315R. If thenumber of operation cycles are greater than, or equal to SUM, then theprocess is advanced to the averaging process of step 310R.

However, if either f_(K) is greater than f_(L) at step 313R, or thenumber of operation cycles is less than SUM, then the tire air pressuredetecting processes after step 301R are again performed.

It should also be noted that while the present embodiment varies thespecification values of the FFT operation based on the vehicle speed V,it is possible to vary the specification according to a vehicle speedvariation rate.

The nineteenth embodiment will be explained with reference to theflowchart of FIG. 50 and waveforms of FIGS. 51, 52 and 53. FIGS. 51(a)and 51(b) show waveforms in a time sequence of vehicle speed v which iscalculated by ECU 4. It should be noted that in the time-based waveform,a low frequency signal component of the wheel speed signal is isolatedby a filter. As shown in FIGS. 51(a) and (b), on a relatively smoothroad, the variation magnitude Δv of the wheel speed is small, and on arough road, the variation magnitude becomes large.

Resonance frequency f_(K) is adapted to detect the resonation phenomenonof the unsprung mass. It should be noted that on rough roads, resonationappears with a large magnitude, thereby permitting easy detection ofresonance frequency f_(K) using various specification values.Accordingly, this embodiment allows for quick detection of decreases intire air pressure while traveling on non-paved roads, or during off-roadtraveling.

Conversely, on smooth roads, resonation appears with a small magnitude,thereby requiring increased specification values for higher detectionprecision.

FIG. 52 shows a map of SMP relative to variation magnitude Δv of thewheel speed, and FIG. 53 shows a map of SON relative to the variationmagnitude Δv of the wheel speed. Both maps are stored in ECU 4.

Wheel speed v is calculated and variation magnitude Δv of the wheelspeed is derived in steps 402S and 403S, respectively. Usingpreliminarily set wheel speed variation magnitudes Δv₁ and Δv₂, the roadsurface is evaluated as rough or smooth.

Then, at step 404S, the specification of the FFT operation is updatedwith values of SMP and SUM based on the value for Δv derived in step403S, where SMP is obtained from the map of FIG. 52 and SUM is obtainedfrom the map of FIG. 53. These values are then used to control the FFToperation as described with respect to the sixteenth embodiment.

It should be noted that multiple wheel speed variation magnitudes Δv₁and Δv₂, which are used for discrimination between the rough road andthe smooth road, are simultaneously set depending upon the road surfacecondition.

The twentieth embodiment is directed at compensating for temperaturechanges within the tire.

The process of this embodiment is shown in FIG. 54, where steps100T˜170T are similar to those of the foregoing embodiments. However,when the process at step 170T is executed, initially calculatedresonance frequency f_(K) is stored as an initial resonance frequency,f_(S). When a tire is heated, the air in the tire is expanded, and theair pressure within the tire increases. Because heating results inchanges in air pressure during a state of constant air volume, it is notpossible to accurately detect tire air pressure based solely on theactual amount of the air in the tire.

Therefore, through steps 180T˜240T, unsprung mass resonance frequencyupper limit value f_(H) and unsprung mass resonance frequency lowerlimit value f_(L) are corrected so that accurate detection of the tireair pressure may be performed irrespective of tire heating.

At step 180T, wheel speed v is compared to predetermined wheel speedV_(T), and rising difference Δf (f_(K) -f_(s)) is compared topredetermined difference Δf₀. It should be noted that predetermineddifference Δf₀ is preliminarily set based on initial resonance frequencyf_(s) to account for heating characteristics of the tire.

When wheel speed v exceeds predetermined speed v_(T), and risingdifference Δf is equal to, or greater than, predetermined differenceΔf₀, the vehicle is determined to be running at high speed and theresonance frequency is increased. Accordingly, the tire can be regardedas heated. For this reason, the process is advanced to step 190T whereflag F is set equal to "1", thereby indicating that f_(H) and f_(L) arein a correction state. Next, the process is advanced to step 200T tocorrect f_(L) and f_(H) due to the temperature change. Namely, f_(L) andf_(H) are corrected through the addition of rising difference Δf toeach.

Otherwise, the process is advanced to step 180T to step 210T where wheelspeed v is compared to predetermined speed v_(T), and rising differenceΔf is compared to predetermined difference f₀. If wheel speed v is equalto, or lower than, predetermined speed v_(T), while rising difference Δfis smaller than predetermined difference Δf₀, the vehicle is determinedto be running at a low speed resulting in decreased resonance frequency.Accordingly, the tire can be regarded as not heating so that the processis advanced to step 230T where flag F is reset to "0", therebyindicating that the correction state is no longer active. At step 240T,unsprung mass resonance frequency upper limit value f_(H) is set equalto unsprung mass resonance frequency upper limit value f_(H) ', andunsprung mass resonance frequency lower limit value f_(L) is set equalto unsprung mass resonance frequency lower limit value f_(L) '.

However, if, in step 210T, wheel speed v is higher than predeterminedspeed v_(T), or if rising difference Δf is larger than predetermineddifference f₀, the temperature of the tire becomes unclear. Therefore,in the present embodiment, the preceding values for f_(L) and f_(H) aremaintained without correction.

Then, the process is advanced to step 220T to check the status of flagF, thereby determining the correction state. If in correction state(F=1), the process is advanced to step 200T to continue correction. Onthe other hand, if not in correction state (F=0), the process isadvanced to step 240T where no correction is performed.

The following examples will demonstrate why the above correction isdependent upon the correction state flag. When the vehicle speed Vexceeds predetermined speed V_(T), rising difference Δf is smaller thanpredetermined difference f₀, and the correction state is enabled (F=1),it can be determined that rising difference Δf is temporarily lowered.However, when not in correction (F=0), the same situation is regarded asbeing caused by an increase in wheel speed due to temporary accelerationof the vehicle.

A timing chart of the processes of steps 180T˜240T, set forth above isshown in FIG. 55. As illustrated in this timing chart, when wheel speedv becomes higher than predetermined speed v_(T), and rising differenceΔf becomes greater than predetermined difference Δf₀, correction isinitiated. However, once initiated, correction is not released untilwheel speed v becomes lower than predetermined speed v_(T), and risingdifference Δf becomes smaller than predetermined difference Δf₀.

Additionally, note that rising difference Δf can be an initially setvalue instead of the derived value (f_(K) -f_(s)).

Furthermore, although the foregoing embodiment performs correction ofboth unsprung mass resonance frequency upper limit value f_(H) andunsprung mass resonance frequency lower limit value f_(L) for each wheelindependently, it may be possible to perform correction for both valuessimultaneously on all wheels while rising difference Δf exceeds thepredetermined difference Δf₀.

For instance, similar to the process of step 200T, rising difference Δfmay be added to both f_(H) and f_(L) which correspond to the wheel inwhich rising difference Δf exceeds predetermined difference Δf₀.Alternatively, for the wheels in which rising difference Δf does notexceed predetermined difference Δf₀, the correction may be performedwith an average value, Δf_(ave), of the rising differences of the wheelΔf.

Until the predetermined vehicle speed is reached, the initial resonancefrequency f_(s) may be determined with either an average value ofresonance frequencies derived, or, with the final value of resonancefrequencies derived, instead of setting it equal to f_(K).

It should be noted that while the above-mentioned embodiment shows anexample of detecting decreases in tire air pressure based only on theresonance frequency of the unsprung mass of the vehicle in thelongitudinal direction, it is possible to detect decreases in the tireair pressure based only on the resonance frequency in the verticaldirection, or, in the alternative based on both of the resonancefrequencies in the vertical and longitudinal directions.

Similar to the twentieth embodiment, the twenty-first embodiment isdirected at correcting unsprung mass resonance frequency upper limitvalue f_(H) and unsprung mass resonance frequency lower limit valuef_(L). However, in addition to the effects gained in the twentiethembodiment, the twenty-first embodiment is directed at preventingstanding wave phenomenon or bursting which are caused by increasedvehicle speeds.

Typically, a tire is rated for a range of vehicle speeds depending uponits grade. Thus, the minimum air pressure (P₀) and maximum air pressure(P_(z)) are based on the vehicle speed and are stored as referencevalues.

However, when the vehicle speed is increased while the tire air pressureis low, bursting or standing wave phenomenon may occur. To prevent this,the overall allowable tire pressure range is increased by raising bothP₀ and P_(z).

Accordingly, the twenty-first embodiment is directed at modifyinginitial unsprung mass resonance frequency upper limit value f_(H) " andinitial unsprung mass resonance frequency lower limit value f_(L) "based on the vehicle speed.

Accordingly, in the twenty-first embodiment, steps 171U˜177U of FIG. 56are added between steps 170T and 180T of the twentieth embodiment.

At step 171U, wheel speed v is compared with first speed v_(Q).

Wheel speed v is not considered excessive when it does not exceed firstspeed v_(Q), and correction is unnecessary. When this is the case, theprocess is advanced to step 172U, where the initial unsprung massresonance frequency upper limit value f_(H) " is set equal to unsprungmass resonance frequency upper limit value f_(H) ', and unsprung massresonance frequency lower limit value f_(L) " is set equal to unsprungmass resonance frequency lower limit value f_(L) '.

However, if wheel speed v exceeds the first speed v_(Q) in step 171U,the process is advanced to step 173U to compare wheel speed v to asecond value, v_(H).

If wheel speed v does not exceed second value v_(H), the process isadvanced to step 174U to perform correction. Namely, initial unsprungmass resonance frequency upper limit value f_(H) " is corrected throughthe addition of ΔQ' to obtain unsprung mass resonance frequency upperlimit value f_(H) ', and initial unsprung mass resonance frequency lowerlimit value f_(L) " is corrected through the addition of correctionvalue ΔQ to obtain unsprung mass resonance frequency lower limit valuef_(L) '.

However, if wheel speed v exceeds second speed v_(H) in step 173U, theprocess is advanced to step 175U to compare wheel speed v to a thirdvalue, v_(V).

If wheel speed v does not exceed third speed v_(V) at step 175U,unsprung mass resonance frequency upper limit value f_(H) ' and unsprungmass resonance frequency lower limit value f_(L) ' are corrected beforethe heating dependent correction by adding correction value ΔH' tounsprung mass resonance frequency upper limit value f_(H) ", and byadding the correction value ΔH to unsprung mass resonance frequencylower limit value f_(L) ".

However, if the vehicle speed exceeds the third speed v_(V), the processis advanced to step 177U to derive unsprung mass resonance frequencyupper limit value f_(H) ' and unsprung mass resonance frequency lowerlimit value f_(L) ' through the addition of correction value Δv' to theunsprung mass resonance frequency upper limit value f_(H) ' andcorrection value Δv to the unsprung mass resonance frequency lower limitvalue f_(L) ".

The result of the foregoing processes of steps 171U˜177U may beillustrated as shown in FIGS. 57(a) and 57(b). When wheel speed v islower than predetermined speed v_(Q), initial unsprung mass resonancefrequency upper limit value f_(H) " is set equal to unsprung massresonance frequency upper limit value f_(H) ', and initial unsprung massresonance frequency lower limit value f_(L) " is set equal to unsprungmass resonance frequency lower limit value f_(L) '. However, when wheelspeed v is increased, f_(H) " and f_(L) " are each corrected so thatf_(H) ' and f_(L) ' are gradually increased.

Correspondingly, as shown in FIG. 57(a), allowable lower limit value P₀and allowable upper limit value P_(z) are also increased to raise theoverall allowable range of the tire air pressure, thereby preventing thebursting or the standing wave phenomenon.

The above-mentioned embodiments are established in view of any singletype of tire. However, if any of the tires are different in type, therating for tire air pressure may also be different. Therefore, thereference value (unsprung mass resonance frequency) for determiningdecreases in tire air pressure may fluctuate corresponding to tire type.

For this reason, the reference value for discriminating abnormalities inthe tire air pressure must be determined depending upon the type of tireused. Results of a study made by the inventors indicate that there aredefinite differences in the tire air pressure, also known as unsprungmass resonance frequency characteristics, between a normal radial tireand a stadless tire (winter tire), as shown in FIG. 58.

Specifically, the fluctuation range of the unsprung mass resonancefrequency of the normal radial tire (hereinafter simply referred to asradial tire) is designated by reference A. This range is higher than thefluctuation range of the unsprung mass resonance frequency of thestadless tire, designated by reference B. Note that this difference maydepend either on the tire manufacture (brand), or on the weight of thewheel to which the tire is equipped.

A_(max) and B_(max) show the upper limit characteristics of thefluctuation in the case where the lightest wheel is employed, andA_(min) and B_(min) show the lower limit characteristics of thefluctuation in the case where the heaviest wheel is employed. Thedifference between the maximum and minimum values is a result of theproportional relationship of unsprung mass resonance frequency f to √k/m(where m is an unsprung mass weight, k is a spring constant of thetire).

Assuming that the air pressure limits for (kg/cm²) alarm due to changein tire air pressure are defined by lower limit P_(L) and upper limitP_(H), then the reference resonance frequency (unsprung mass resonancefrequency) f_(L) for determining the radial tire air pressure becomesf_(RA). Similarly, the reference resonance frequency f_(L) of thestadless tire becomes f_(ST). In this case, the minimum air pressure, asdefined in JIS standard (1.4 kg/cm2) can be used for P_(L), and themaximum air pressure, as defined in JIS standard (2.5 kg/cm2) can beused for P_(H).

In the twenty-second embodiment, using various combinations of twoswitches, 6a and 6b, the type of the tires equipped on the two frontwheels and two rear wheels can be determined. Then, reference resonancefrequencies can be correspondingly set. Therefore, even when the type ofthe tires are changed, the air pressure condition of the tires can beaccurately detected.

An example of the processes performed in the twenty-second embodimentare shown in the flowcharts illustrated by FIGS. 59 and 60.

At step 101V, the status of flag F is checked, where it is reset to "0"only when the ignition switch has been turned OFF. Accordingly, theresult of step 101V is negative only immediately after signal processingbegins so that the process proceeds to step 102V. Otherwise, theprocesses of this embodiment are foregone.

At step 102V, determination is made whether both of selection switches,6a and 6b, are in ON state. If both are in ON state, judgment is made instep 105V that stadless tires are used on all four wheels. Therefore,reference resonance frequency f_(L) is set equal to f_(ST) for all fourwheels at step 105aV.

If both selection switches are not in the ON state at step 102V, theprocess is advanced to step 103V to determine whether both selectionswitches 6a and 6b are in the OFF state. If both of the switches areOFF, judgment is made in step 106V that radial tires are used on allfour wheels. Therefore, reference resonance frequency f_(L) is set equalto f_(RA) for all four wheels at step 106aV.

If both selection switches are not in the OFF state, the process isadvanced to step 104V. If in step 104V, selection switch 6a isdetermined to be in the OFF state, selection switch 6b must necessarilybe ON from the results of prior tests. If this is the case, the processadvances to step 107V, where judgment is made that radial tires areequipped on the two front wheels, and stadless tires are equipped on thetwo rear wheels. Therefore, reference resonance frequency f_(L) for thetwo front wheels is set equal to f_(RA), and reference resonancefrequency f_(L) for the two rear wheels is set equal to f_(ST) in step107aV.

If in step 104V, selection switch 6a is determined to be in the ONstate, judgment is made at step 108V that stadless tires are equipped onthe two front wheels, and radial tires are equipped on two rear wheels.Therefore, at step 108aV, resonance frequency f_(L) for the two frontwheels is set equal to f_(ST), and reference resonance frequency f_(L)for the two rear wheels is set equal to f_(RA).

Consequently, only one portion of the processes described by steps105V˜108V are performed. Further, the processes subsequent to step 108V,illustrated in FIG. 60, are explained with respect to the case wherestadless tires are equipped on two front wheels, and radial tires areequipped on two rear wheels.

At steps 109V˜117V, similar processes to those of the former embodimentsare performed.

However, at step 118V, when derived resonance frequency f_(K) is lowerthan, or equal to, reference resonance frequency f_(ST) for the stadlesstire, or when f_(K) is lower than, or equal to, reference resonancefrequency f_(RA) for the radial tire, it is determined that the airpressure of at least one tire is below the allowable lower limit value.Thus, the process is advanced to step 119V, where an alarm is displayedto the driver on display portion 5. Otherwise, the process is returnedto step 101V.

It should be noted that although the foregoing embodiment employs f_(ST)and f_(RA) as reference resonance frequencies, it is possible to use adifference (f_(ST0) -f_(KST) or f_(RA0) -f_(KRA)) between the resonancefrequency f_(ST0) or f_(RA0) at the normal air pressure, or to usecalculated resonance frequencies f_(KST) and f_(KRA), as referenceresonance frequencies.

In the twenty-third embodiment derived resonance frequency f_(K) is setequal to reference resonance frequency f_(K0) when setting switch 16 isturned on by the driver after a tire changing operation. Therefore, thetire air pressure can be detected with high precision irrespective ofthe type of new tires used.

The twenty-third embodiment will be explained with reference to theflowchart of FIG. 61 as well as FIGS. 62 and 63. FIG. 62 is arelationship between the resonance frequency and the tire air pressure.

The processes of steps 201W-208W, are similar to those of thetwenty-second embodiment. However, at step 209W, the status of flag F iscompared to "1", where the flag F is reset to "0" only when the ignitionswitch is turned OFF. Therefore, the process is advanced to step 210Wonly when first tested; otherwise, the process is advanced directly tostep 211W. At step 210W, the state of setting switch 16 is determined,where this switch is shown in FIG. 63.

If switch 16 is OFF in step 210W, the lowering difference betweenresonance frequency f_(X), and reference resonance frequency f_(K0) iscompared to reference difference Δf in step 211W, where referencedifference Δf is between the above-mentioned frequency, f_(K0), and theresonance frequency, f_(L). Further, this reference differencecorresponds to the tire air pressure lowering alarm limit as shown inFIG. 62.

For example, as set forth by step 211W, if the lowering difference isless than, or equal to, the reference difference [(f_(K0) -f_(K))≦Δf],the tire pressure is determined to be within the allowable limit, andthe process is redirected to step 201W. On the other hand, if thelowering difference is greater than the reference difference [(f_(K0)-f_(K))>Δf], the tire pressure is determined to be below the allowablevalue and the process is advanced to step 212W where an alarm isdisplayed for the driver on display portion 5.

When switch 16 is in the ON state at step 210W, initial resonancefrequency f_(K) is set equal to reference resonance frequency f_(K0) foreach of the four wheels independently, at step 213W. Then, at step 214W,flag F is set to "1", and the process returns to step 201W.

Accordingly, after flag F has been set to "1", tire air pressuredetection is performed by comparing the difference between newly setreference resonance frequency f_(K0) and sequentially derived resonancefrequency f_(K), with difference Δf regardless of the state of switch16, where reference difference Δf is measured between referenceresonance frequency f_(K0) and resonance frequency f_(L). It should benoted that while the reference resonance frequency f_(K0) can be setindependently for each of the four wheels as mentioned above, it is alsopossible to set reference resonance frequency f_(K0) equal to (1) anaverage value of resonance frequencies f_(K) derived with respect toeach of the four wheels, (2) an average value of two wheels excludingthe maximum and minimum values, or (3) the maximum or minimum value ofresonance frequencies f_(K), for each of the respective four wheels.

The twenty-fourth embodiment is directed at performing a similar processas described in the twenty-third embodiment, except that theabove-mentioned setting switch 16 is neglected. By setting resonancefrequency f_(K), which is derived immediately after starting the tirepressure detecting process, equal to reference resonance frequencyf_(K0), any decrease in tire air pressure which occurs during operationis detected irrespective of the tire type.

The twenty-fourth embodiment will be explained with reference to theflowchart of FIG. 64, wherein step 210W of the twenty-third embodimenthas been deleted as if switch 16 were in the ON state.

Therefore, at step 209X, determination is made whether flag F is set to"1", or not If it is not set to "1", the process is advanced to step213X so that resonance frequency f_(K) is set equal to referenceresonance frequency f_(K0). On the other hand, when the answer ispositive, the process is advanced to step 211X for further processing.

It should be noted that as in the twenty-third embodiment, referenceresonance frequency f_(K0) can be set in the above-mentioned manner of(1)˜(3).

The twenty-fifth embodiment uses an effective rolling radius and theunsprung mass resonance frequency to determine the tire type. Namely, asshown in FIG. 65, effective rolling radius r_(s) and unsprung massresonance frequency f_(s) are linearly related based on the type of thetire, where line x is normal radial tire, line y is stadless tire, andline z is a low profile tire, each of which corresponds to thepreviously explained types of tire. Using a map of this type, the typeof the tire can be determined with r_(s) and f_(s). For this reason, atire changing judgment map, similar to that of FIG. 65, is stored in ECU4.

The process used to determine effective rolling radius r_(s) andunsprung mass resonance frequency f_(s), as well as subsequentdetermination of tire type, is illustrated by the flowcharts of FIGS. 66and 67.

At steps 101Y and 102Y, wheel speed v is derived on the basis of thesignal from the wheel speed sensor, and flag F is checked, before thetire has been subjected to centrifugal force. It should be noted thatwheel speed v is calculated by waveshaping the output signal of thewheel speed sensor, and by dividing the number of the resultant pulseswith a corresponding period.

If flag F is not set to "1", the process is advanced to step 103Y. Atstep 103Y, vehicle speed V may be detected by means of a doppler-typevehicle speed meter, or a rotational speed of a transmission rotaryshaft. Then, at step 104Y, tire load radius r_(s) is derived on thebasis of both vehicle speed V and wheel speed v.

At steps 105Y and 106Y, an FFT operation is performed with respect tothe wheel speed. The process is repeated until the number of operationcycles of the frequency analysis, K, reaches a predetermined number, K₀,at which time it is advanced to step 107Y.

At steps 107Y and 108Y, the results of the frequency analysis areaveraged, and unsprung mass resonance frequency f_(s) is calculatedbased on this average.

At step 109Y, a map (FIG. 65) is used in conjunction with effectiverolling radius r_(s) and unsprung mass resonance frequency f_(s) todetermined the type tire. At step 110Y, alarming reference values, f_(L)and f_(H), are determined and stored based on the tire type, where thevalues for each tire are shown on the map of FIG. 68 as f_(La), f_(Lb),f_(Lc), f_(Ha), f_(Hb), f_(Hc).

Only then is flag F is set to "1" at step 111Y. Thus, theabove-mentioned steps 103Y˜110Y for determining tire type are executedonly immediately after vehicle ignition. In practice, foregoing step110Y is executed only when it is determined that all four wheels or atleast the two drive wheels have been changed in step 109Y.

It should be noted that each process shown in FIG. 67 has been describedin the foregoing embodiments.

It should also be noted that the tire type discrimination of step 109Ymay be performed with a regional map as illustrated in FIG. 69, insteadof the linear map shown in FIG. 65. Accordingly, discrimination betweennormal radial, stadless, and low-profile tires is made based on theregion mapped to by effective rolling radius r_(s) and unsprung massresonance frequency f_(s).

Furthermore, determination of the tire type can be performed byemploying the matrix shown in TABLE 1. Namely, based on variation ineffective rolling radius r_(s) and unsprung mass resonance frequencyf_(s), multiple matrices are formed based on the tire types.

                  TABLE 1                                                         ______________________________________                                                    Tire Load Radius (r.sub.s)                                                    Decreased                                                                              Unchanged Increased                                      ______________________________________                                        Unsprung                                                                              Increased c          c       a                                        Mass              (Low       (Low    (Normal                                  Resonance         Profile    Profile Tire)                                    Frequency         Tire)      Tire)                                                    Unchanged c          a       b                                                          (Low       (Normal (Stadless                                                  Profile    Tire)   Tire)                                                      Tire)                                                               Decreased a          b       b                                                          (Normal    (Stadless                                                                             (Stadless                                                  Tire)      Tire)   Tire)                                    ______________________________________                                    

For instance, if normal radial tires are used, a decrease in theunsprung mass resonance frequency, which is caused by decreased tire airpressure, will lead to a decrease in the effective rolling radius.Conversely, an increase in tire air pressure results in an increase inthe effective rolling radius corresponding to an increase in theunsprung mass resonance frequency. These characteristics are designatedby an "a" in the matrix shown in TABLE 1.

Rubber used to construct stadless tires is softer and this results ingenerally lower unsprung mass resonance frequencies. The correspondingbehavior is shown in blocks designated with a "b" in TABLE 1.

On the other hand, the low profile tire generally has a high tire springconstant. Therefore, the unsprung mass resonance frequency is generallyhigh, and the corresponding behavior is shown in blocks designated witha "c" in TABLE 1.

It should be noted that when both r_(s) and f_(s) are either increasedor decreased, it is difficult to discriminate between tire types.However, because changes in tire pressure are generally different, thetire type can often be determined by aggregating the results ofdiscrimination of the other wheels.

For instance, when the unsprung mass resonance frequency and theeffective rolling radius are decreased simultaneously at two or four ofthe wheels, it can be inferred that the tires have been changed tostadless tires. Conversely, when both or all four tires have risen,judgment can be made that the tires have been changed to low profiletires.

Therefore, with the present embodiment, effects similar to those of theforegoing embodiment can be achieved.

It should be noted that an optimal air pressure value for the normalradial tire, or a value measured immediately before the vehicle stopscan be used for the above-mentioned reference values, r₀ and f₀.

Next, the twenty-sixth embodiment will be discussed. Decreased tire airpressure may be caused by natural leakage or puncture. Generally,however, punctures cause these decreases. It is rare that punctures areexperienced by both left and right wheels simultaneously. However,changing of the tire or wheel materials results in a variation of theunsprung mass weight which affects the vertical and longitudinalresonance frequency components in the unsprung mass of the vehicle.

By deriving and comparing the resonance frequencies of the left andright wheels with respect to each of the drive wheels and driven wheels,judgment can be made that the tire air pressure has decreased in thetire which has the lowest resonance frequency. That is, only if there isdefinite difference between the resonance frequencies. In the presentembodiment, control is performed in consideration of the above.

The foregoing process is illustrated in the flowcharts shown by FIG. 70,where steps 101Z˜108Z are similar to those described in previousembodiments, and step 109Z is further described using the flowchart ofFIG. 71.

At step 201Z, resonance frequency f_(L), which is derived with respectto the left side wheel of the front or rear, is compared with theresonance frequency f_(R), which is derived with respect to the rightside wheel. Then, depending upon the results of comparison between f_(L)and f_(R), step 202Z or 203Z is performed to set the higher resonancefrequency as f_(MAX), and the lower resonance frequency as f_(MIN).

When the unsprung mass weight is varied, the relationship between theresonance frequency and the tire air pressure fluctuates, as shown bythe hatched region in FIG. 72. Therefore, any one of a number of tireair pressures may result from one resonance frequency. Thus, in step204Z, a minimum value of the tire air pressure, P_(MIN), whichcorresponds to f_(MIN), is derived from a relationship between theresonance frequency (Hz) and the tire air pressure (kg/cm²).

Then, the process is advanced to step 205Z, where minimum value P_(MIN)is compared with threshold level P_(TH) to detect abnormal decreases intire air pressure.

If P_(MIN) is less than P_(TH), the process jumps to step 209Z todisplay an alarm indicative of the abnormal decreases in tire airpressure on display portion 5. This process is a preventive measure forthe case where the tire air pressures of both of the left and rightwheels are lowered simultaneously.

Otherwise, if P_(MIN) is greater than, or equal to P_(TH) at step 205Z,the process is advanced to step 206Z, where difference Δf is derivedfrom resonance frequencies f_(MAX) and f_(MIN) of the left and rightwheels. As set forth above, when the unsprung mass weight is varied viatire variation, wheel material or so forth, the characteristics betweenthe resonance frequency and the tire air pressure are also varied.

As shown in FIG. 73, Δf_(A) corresponds to the difference between normalresonance frequency f_(AN) and abnormal resonance frequency f_(AS),where f_(BN) corresponds to normal tire air pressure P_(N), andresonance frequency f_(BW) corresponds to the abnormally decreasing tireair pressure P_(W), as shown by characteristic curve (B). Further,Δf_(B) corresponds to the difference between normal resonance frequencyf_(BN) and abnormal resonance frequency f_(BW), where f_(BN) correspondsto normal tire air pressure P_(N), and resonance frequency f_(BW)corresponds to the abnormally decreasing tire air pressure P_(W), asshown by characteristic curve (B). Because difference Δf_(A) anddifference Δf_(B) may be different, abnormal decreases in tire airpressure may be erroneously detected by simply evaluating one or theother difference as Δf.

Thus, threshold level f_(TH) is unconditionally determined for judgmentof abnormally low tire air pressure, where f_(TH) is the differencebetween the resonance frequencies.

If the variation of the unsprung mass coefficient factor in the left andright wheels is caused only by the difference of the tire air pressuresat those wheels, then the unsprung mass coefficient factors other thanthe tire air pressure, can absorb the influence for the resonancefrequency as follows. Characteristic charts, as shown in FIG. 74, of therelationship between the maximum resonance frequency and a difference ofresonance frequencies for each tire type can be detected. For instance,the normal tire air pressure (e.g. 2.0 kg/cm²) and alarming tire airpressure (e.g. 1.4 kg/cm²) are derived with respect to variouscombinations of tires and wheels. Then, using the characteristic linesindicated by this data, the resonance frequency for each tire type isused to determine the threshold corresponding to the other tire type.

It should be noted that the characteristic chart shown in FIG. 74 isstored in ECU 4. Accordingly, at step 207Z, threshold level f_(TH) isobtained from the map stored in ECU 4 with respect to resonancefrequency f_(MAX), and is regarded as normal tire air pressure.

Then, at step 208Z, resonance frequency difference Δf is compared withnew threshold level f_(TH). If Δf is greater than, or equal to f_(TH),the process is advanced to 209Z where an alarm is displayed on displayportion 5 to indicate an abnormal decrease in the tire air pressure. Onthe other hand, if Δf is less than f_(TH), the process simply returns toother processing.

It should be noted that, depending upon the vehicle speed and travelingcondition, a specific tire air pressure may be determined to be eitherdangerous or not dangerous. However, using a map of many characteristiccurves which take into account the vehicle speed and travellingcondition (FIG. 75), threshold level f_(TH) may be accurately derived.

It should also be noted that in the foregoing, the decreases in tire airpressure can be determined by employing resonance frequency f_(MAX)instead of resonance frequency _(MIN). Selection of either f_(MIN) orf_(MAX) is made by taking the degree of the tire air pressure decreasefor the left and right wheels into consideration. The relationshipbetween the resonance frequency and the tire air pressure shown in FIG.72, is preliminarily stored in the form of a map in ECU 4.

The foregoing embodiment can improve reliability by avoiding erroneousdetection of lowering abnormalities of tire air pressure. Because therelationship between variation magnitude Δf and the variation magnitudeof the tire air pressure is affected by the unsprung mass coefficientfactor, threshold level f_(TH) can be corrected using resonancefrequency f_(MAX) which is regarded to be normal tire resonancefrequency.

On the other hand, by setting f_(MAX) or f_(MIN) as the threshold valuefor judgment, natural leakage may be detected when the tire air pressureof left and right wheels are lowered simultaneously. Thus, with respectto fluctuation of the characteristics between the resonance frequencyand the tire air pressure which depend upon the type of tire and wheelused, the set threshold value for judgment may be adjusted by selectingfrom among f_(MAX) and f_(MIN).

The twenty-seventh embodiment is directed at improving detectionreliability by using a two-stage judgment to detect for decreases in thetire air pressure. Specifically, after a variation rate of resonancefrequency f_(K) is obtained for a unit period, the variation rate iscompared to a judgment value. Then, the number of cycles in which thevariation rate is less than this judgment value are compared topredetermined value, Mo. Only after both predetermined values have beenexceeded will the alarm be set. Thus, temporary fluctuations whicherroneously result in detection of decreased pressure are avoided.

The foregoing embodiment is illustrated by the flowchart of FIG. 76,where steps 101α˜108α are similar to those in former embodiments.

However, at step 109α, derived resonance frequency f_(K) is compared topredetermined air pressure lowering discrimination value f_(L).

If f_(K) is greater than f_(L), then the process is advanced to step114α where counter m is reset and the process is returned to step 101α.

However, if f_(K) is less than, or equal to, f_(L), the process isadvanced to step 110α where resonance frequency variation rate df_(K) iscompared with judgment value (Δf_(K) /Δt) to determine the degree ofdecrease in the tire air pressure. It should be noted that Δf_(K) is thedifference between the current calculated resonance frequency andprevious calculated resonance frequency, and that Δt is the elapsedperiod therebetween.

When variation rate df_(K) is greater than, or equal to, foregoingjudgment value (Δf_(K) /Δt), the tire air pressure is considered to begradually decreasing. Therefore, the process is advanced to step 111α toincrement counter m. Subsequently, at step 112α, it is determinedwhether derived variation rate of the derived resonance frequency hasmaintained a level lower than judgment value for more than m_(o) cycles.

If it has been lower for m_(o) cycles, the answer at step 112α ispositive, and the process is advanced to step 113α to display the alarmindicating low tire air pressure for the relevant tire.

On the other hand, when it has not been lower for m_(o) continuouscycles, the answer at step 112α is negative, and the process is returnedto step 101α.

Further, when the variation rate df_(K) of the resonance frequency isless than judgment value (Δf_(K) /Δt) in step 110α, judgment is madethat the tire air pressure is abruptly lowered due to occurrence ofabrupt decrease in tire air pressure. Therefore, the process jumps tostep 113α to permit alarming display of the foregoing content.

It should be noted that, once initiated, the present embodimentmaintains alarming display until the vehicle stops. Then, uponrestarting the vehicle, if the initial resonance frequency f_(K) ishigher than the air pressure lowering judging value, the tire airpressure lowering detection state is released to terminate alarmingdisplay. However, if the initial resonance frequency f_(K) is lowerthan, or equal to, the air pressure lowering judgment value, thealarming display is maintained until the next stop of the vehicle torepeat the foregoing steps.

The twenty-eighth embodiment is directed at determining a referencevalue for judging abnormality in tire pressure based on characteristicsof the tire and wheel in the manner different from the twenty-seventhembodiment.

As described in the twenty-third embodiment at FIG. 62, the tirepressure, unsprung resonance frequency characteristic is illustrated asshown in FIG. 77.

When the air pressure of the tire which is mounted on the lightest wheelis lowered, the air pressure (kg/cm2) lower limit value and upper limitvalue are taken as P_(L) and P_(H), respectively. Correspondingly, thereference/unsprung resonance frequency f_(L) for judging the decrease inair pressure in the radial tire is indicated by F_(RAL), and thereference resonance frequency f_(L) for the stadless tire is indicatedby f_(STL). It should be noted that P_(L) and P_(H) may be set equal tothe minimum and maximum air pressures recited by the JIS, namely 1.4gk/cm² and 2.5 kg/cm², respectively.

In addition, because f_(K0) is used to determine the type of tireemployed, it is desirable to set its value to a value centered betweenf_(RAD) and f_(STL), where f_(RAD) and f_(STL) are the resonancefrequency equivalents to upper limit pressure P_(H) of the stadlesstire, and the lower limit pressure P_(L) of the radial tire,respectively.

The signal processing performed by ECU 4 in the twenty-eighth embodimentwill be described with reference to the flowchart of FIG. 78.

It should be noted that the processes before step 170α are similar tothose described by steps 100A to 160A of the first embodiment.

However, in step 170β, longitudinal resonance frequency f_(K) andvertical resonance frequency f_(R) of the unsprung vehicle aredetermined based on the smoothed results of the FFT operation (shown inFIG. 79). At step 180β, the status of flag F is checked, where it isreset to "0" only after the ignition key is turned off.

If flag F does not equal "1", at step 180β, the process proceeds to step190β where resonance frequency f_(K) is compared to resonance frequencyf_(K0) in order to determine tire type.

As shown in FIG. 77, the operated resonance frequency f_(K) is less thanthe resonance frequency f_(K0) when the tire type is a radial tire andthe tire pressure is very low, and when the tire type is considered tobe stadless. Consequently, when resonance frequency f_(K) is lessresonance frequency f_(K0), the tire type can not be determined based onresonance frequency f_(K0) alone.

Wheel speeds (gains) v_(R) and v_(K), which respectively correspond toresonance frequencies f_(R) and f_(K), are shown in FIGS. 80(a) and80(b). As shown, when the tire pressures of the radial and stadlesstires are decreased, the orientation of gains v_(R) and v_(K) for theradial tire (see FIG. 80(a)) are reversed, while the orientation ofgains v_(R) and v_(K) for the stadless tire (see FIG. 80(b)) do notchange.

Consequently, using a ratio between the gains (v_(R) /v_(K)), these tiretypes can be discriminated from each other.

For this reason, when resonance frequency f_(K) is less than, or equalto, f_(K0) at step 190β, the process proceeds to the step 200β, wheregain ratio v_(R) /v_(K) is calculated and compared with predeterminedvalue α. Then, if gain ratio v_(R) /v_(K) is greater than, or equal to,α, it is determined that the tire is stadless and the process isadvanced to step 220β.

However, if either resonance frequency f_(K) is greater than f_(K0) atstep 190β, or gain ratio v_(R) /v_(K) is less than αat step 200β, it isdetermined that the tire is radial, and the process is advanced to step210β.

At steps 210β and 230β, the tire weight (wheel weight) M is determinedbased on the resonance frequency f_(R). A map is then used to determinejudgment resonance frequency f_(L) corresponding to tire pressurereduction warning pressure P_(L).

If the tire spring constant and the unsprung weight are taken as k andm, respectively, then the resonance frequency f_(R) is calculated by thefollowing operational equation: ##EQU1##

Because unsprung weight m is a constant value which is determinedthrough vehicular data, when tire spring constant k is specified throughthe tire air pressure, resonance frequency f_(R) is based on tire weightm. Thus, for example, three values are determined for vertical resonancefrequency f_(R), namely, f_(RA) corresponding to light tire weight,f_(RB) corresponding to intermediate tire weight, and f_(RC)corresponding to light tire weight (see FIG. 81). Then, thepressure-resonance frequency characteristics are respectively calculated(see FIG. 82), and the resultant map is recorded in ROM of ECU 4.

The values for judgment resonance frequencies f_(L1), f_(L2), and f_(L3)corresponding to tire pressure reduction warning pressure P_(L) are thendetermined for resonance frequencies f_(RA), f_(RB), and f_(RC),respectively.

Alternatively, if it is determined in steps 190-200β that stadless tireshave been employed, the processes shown by steps 220β and 240β areperformed, where these processes are similar to those of steps 210β and230β.

When the determination of the tire type has been completed, judgmentresonance frequency f_(L) corresponding to the tire pressure reductionwarning pressure P_(L) is determined using the map shown in FIG. 82, andflag F is set at "1" in step 250β. Then, at step 260β, operatedresonance frequency f_(R) is compared with resonance frequency f_(L).

When f_(R) is less than, or equal to f_(L), the tire air pressure isdetermined to be excessively low, and the process proceeds to step 270β,where a warning is displayed to the driver by display unit 5.

However, when f_(R) is less than f_(L), the process is returned to 100A.

It should be noted that because flag F is set equal to "1" after thisprocess has been completed, the processes from step 190β to 250β areomitted after the first cycle.

In the above-described embodiment, the tire type is determined based ona gain ratio between vertical resonance frequency f_(R) and longitudinalresonance frequency f_(k). It should be noted that the determination oftire type may also be performed on the basis of either a large or smallrelationship between resonance frequencies f_(R) and f_(k), or thedeviation therebetween.

Further, the judgment for the above wheel weight may be performed on thebasis of the variation in maximum vertical resonance frequency f_(R).

Consequently, because determination of the tire type is automaticallyperformed, and because the judgment value used to compare the tire airpressure can be set according to the weight of the wheel used to mountthe tire, it is possible to achieve very accurate detection of the tirepressure state.

When a vehicle is operated on a rough road, forces are applied to boththe tires and the suspension. Therefore, even if the tire pressure isconstant, the vertical or longitudinal resonance frequencies under theunsprung state are decreased due to the influence of non-linearcharacteristics of bushings and vibration-proof rubber which are used inthe suspension.

Additionally, when the vehicular speed is lowered, the signal level(gain) used to determine resonance frequency is reduced. Thus, whenbraking or slowing down, it may become impossible to accurately detectthe resonance point.

The twenty-ninth embodiment is directed at handling the above-describedproblems through the processes illustrated in FIG. 83.

The processes before step 130τ are similar to steps 100A to 120A of thefirst embodiment, illustrated in FIG. 10. In subsequent steps 130τ and140τ, an FFT operation and a data selection process are executed.Specifically, in the data selection processing, a selection lower limitdetermination value v_(L) and a selection upper limit determinationvalue v_(H) are determined based on the waveform of the wheel speed.These values are compared to peak value v_(P) within predeterminedfrequency range f_(1-f) ₂.

If v_(P) is less than, or equal to v_(L) (FIG. 84(b)), or if v_(P) isless than, or equal to v_(H) (FIG. 84(c)), then the results of the FFToperation performed on corresponding portions (A) and (B) are not usedto determine resonance frequency f_(K).

In the above step 140τ, data selection is performed based on the upperand lower limit values, v_(H) and v_(L). However, even in the data afterselection (portion (C) in FIG. 84(a)), the magnitude of the gainresulting from each FFT operation is variable, as shown in FIG. 85. As aresult, both the number of averaging processes and the time required tocalculate resonance frequency, f_(K), are increased.

Consequently, in step 150τ, the gain (magnitude) of the wheel speedsignal is adjusted by multiplying the result of each FFT operational bya coefficient K₁, K₂, . . . K_(i). Therefore, the peak values within thepredetermined frequency range (f₁ to f₂) are set equal to thepredetermined value, v_(PK), as shown in the resultant FFT operationwaveform.

Finally, in subsequent step 160τ, the FFT operational number N isincremented, and the process is advanced to perform steps similar tothose after step 140A of the first embodiment.

In the thirtieth embodiment, a different data selection process and gainadjustment process for the time-waveform of the wheel speed v areperformed. This process is shown by the flowchart of FIG. 86.

At step 100A, the wheel speed is calculated. Then, in the data selectionprocess of step 111γ, the selection of lower limit judgment value |v_(L)'| and the selection of upper limit judgment value |v_(H) '| are made asshown in FIG. 87. Then, only the time-waveform of wheel speed v having amagnitude within the range of (-v_(H) ' TO -v_(L) '), or (-v_(L) ' TO-v_(H) ') is considered.

Correspondingly, the gain adjustment in step 112γ, is performed bymultiplying wheel speed v within specified time Δt' by coefficient k₁ '.Then, after the data selection process, the peak value within specifiedtime Δt', which is designated by v_(p) ', is set equal to apredetermined value, as shown in FIG. 88. The processes after step 112γare then performed, as described with respect to the first embodiment,to detect decreases in the tire pressure as described with respect tothe first embodiment.

In the above-described twenty-ninth and thirtieth embodiments, byexecuting a data selection process, resonance frequency f_(K) fordetecting the tire pressure can be determined without being lowered.

Further, since the gain adjustment process is performed, even when thefrequency characteristic is changed, the magnitude of the gain is notvariable, as result of each FFT operation. It is therefore impossible toreduce the number of averaging processes required for calculating theaverage of the FFT operational results. Consequently, in thetwenty-ninth and thirtieth embodiments, it is difficult to rapidlydetect lowering of the tire pressure.

It should also be noted that in each above-mentioned embodiments, thevalue of the tire air pressure, as well as the abnormal alarm of thetire air pressure may be displayed directly.

Although the embodiments have been disclosed in detail, the presentinvention should not be limited to these embodiments. For instance, inFIG. 4, it is possible to detect the tire air pressure on the basis ofvariation of gain at a specific frequency or variation of the frequencyat a specific gain.

As set forth above, according to the present invention, thepredetermined frequency component in the tire vibration frequencycomponent varies according to variation of the spring constant of thetire. Therefore, the air pressure condition of the tire is detectedbased on the variation of the frequency component of a tire.Consequently, the vehicular occupant can monitor the tire air pressurewhile traveling in the vehicle. In addition, by employing a device whichadjusts the tire air pressure during travel, driving performance can besignificantly enhanced.

What is claimed is:
 1. A tire air pressure detecting devicecomprising:output means being installed on a vehicle, for outputting asignal including a vibration frequency component of a tire while saidvehicle is moving; extracting means for extracting a resonance frequencycomponent from said signal including said tire vibration frequencycomponent; and detecting means for detecting a tire air pressurecondition based on said resonance frequency component.
 2. A tire airpressure detecting device as set forth in claim 1, wherein saidoutputting means comprises a wheel speed sensor for generating a signalcorresponding to a rotation speed of a wheel.
 3. A tire air pressuredetecting device as set forth in claim 1, wherein said extracting meansextracts said resonance frequency component from said signal output bysaid output means based on vibrations of an unsprung mass of saidvehicle which are generated in at least one of a vertical direction anda longitudinal direction.
 4. A tire air pressure detecting device as setforth in claim 1, wherein said detecting means preliminarily stores aresonance frequency value as a reference resonance frequency, anddetects lowering of said tire air pressure condition based on avariation magnitude of said extracted resonance frequency relative tosaid stored resonance frequency value.
 5. A tire air pressure detectingdevice as set forth in claim 1, wherein said detecting meanspreliminarily stores a relationship between said tire air pressure andsaid resonance frequency component, and predicts said tire air pressurefrom said extracted resonance frequency based on said storedrelationship.
 6. A tire air pressure detecting device as set forth inclaim 1, further comprising an alarming means for alarming a driver whena decrease in said tire air pressure condition to a level below a lowerlimit air pressure is detected by the detecting means.
 7. A tire airpressure detecting device as set forth in claim 1, further comprisingremoving means for removing higher order components from said signalcontaining said tire vibration frequency component, said higher ordercomponents including noise components appearing at frequencies which areinteger multiples of a frequency corresponding to a number of wheelrotations within a unit period of time.
 8. A tire air pressure detectingdevice as set forth in claim 1, wherein said extraction means includesan extraction period varying means for modifying an extraction period.9. A tire air pressure detecting device as set forth in claim 1, whereinsaid detecting means includes correcting means for correcting saidreference value used to detect low tire air pressure based on a vehicletraveling speed.
 10. A tire air pressure detecting device as set forthin claim 1, which further comprises tire type selection means forselecting a type of tire which is equipped on said vehicle.
 11. A tireair pressure detecting device as set forth in claim 1, which furthercomprises tire type selection means for selecting a type of tire whichis equipped on said vehicle, where said tire type selection means is aswitch to be operated by a vehicular occupant.
 12. A tire air pressuredetecting device as set forth in claim 1, which further comprises tiretype selection means for selecting a type of tire which is equipped onsaid vehicle, where said tire type selection means selects said type oftire based on a tire load radius.
 13. A tire air pressure detectingdevice as set forth in claim 1, wherein said detecting means comprisesresonance frequency difference deriving means for deriving a differencebetween resonance frequencies for left and right wheels from saidrespective resonance frequencies, and judgment means for comparing saidderived difference in resonance frequency with a judgment value.
 14. Atire air pressure detecting device as set forth in claim 1, wherein saiddetecting means determines tire air pressure abnormalities based on saidresonance frequency component and outputs an abnormality signal when anabnormal tire air pressure is detected for more than a predeterminednumber of processing cycles.
 15. An apparatus for detecting a tire airpressure according to claim 1, further comprising tire type selectionmeans for selecting a type of tire which is equipped on said vehicle,wherein said tire type selecting means determines said tire type basedon a ratio between gains of a resonance frequency based on vibrations ina vertical direction and gains of a resonance frequency based onvibrations in a longitudinal direction.
 16. An apparatus for detecting atire air pressure according to claim 1, further comprising tire typeselection means for selecting a type of tire which is equipped on saidvehicle, wherein said tire type selecting means determines said tiretype based on a deviation between a resonance frequency based onvibrations in a vertical direction and a resonance frequency based onvibrations in a longitudinal direction.
 17. An apparatus for detectingtire air pressure according to claim 1, further comprising a weightjudging means for judging a weight of a wheel which is mounted to saidvehicle.
 18. An apparatus for detecting a tire air pressure according toclaim 1, further comprising a judging means for judging a weight of awheel which is mounted on said vehicle, wherein said judging meansdetermines said weight of said wheel based on an amount of variation ina resonance frequency based on vibrations in a vertical direction. 19.An apparatus for detecting a tire air pressure according to claim 1,further comprising a signal selecting means for selecting the signalcontaining the resonance frequency component which is used to determinea resonance frequency of said tire.
 20. An apparatus for detecting atire pressure according to claim 1, further comprising signal selectingmeans for selecting signal containing the resonance frequency componentwhich is used to determine a resonance frequency of said tire, and asignal adjusting means for adjusting a signal level of said signalcontaining said resonance frequency component selected by said signalselecting means.
 21. A tire air pressure detecting devicecomprising:output means being installed on a vehicle, for outputting asignal including a vibration frequency component of a tire while saidvehicle is moving; extracting means for extracting a resonance frequencycomponent from said output signal; storing means for storing at least areference value, said reference value being based on a resonancefrequency at normal tire air pressure; and detecting means for detectinga tire air pressure condition by comparing said extracted resonancefrequency component with said stored reference value.
 22. A tire airpressure detecting device as set forth in claim 21, wherein said storingmeans preliminarily stores a resonance frequency value as said referencevalue, and wherein said detecting means detects lowering of said tireair pressure based on a variation magnitude of said extracted resonancefrequency component relative to said resonance frequency value.
 23. Atire air pressure detecting device as set forth in claim 21, whereinsaid storing means preliminarily stores a relationship between tire airpressure and resonance frequency, and said detecting means predicts saidtire air pressure from said extracted resonance frequency componentbased on said stored relationship.
 24. A tire air pressure detectingdevice comprising:output means being installed on a vehicle, foroutputting a signal including a vibration frequency component of a tirewhile said vehicle is moving; extracting means for extracting aresonance frequency component from said output signal; and detectingmeans for detecting a tire air pressure condition based on saidresonance frequency component of said signal, said detection being madeindependent of resonance frequency components corresponding to othertires on said vehicle.